Paper pigments



United States atent 2,943,970 PAPER PIGMENTS Edward M. Allen, Doylestown, Ohio, assignor to Columbia-Southern Chemical Corporation, Allegheny County, 'Pa., a corporation of Delaware No Drawing. Filed Apr. 14, 1959, Ser. No. 806,212

34 Claims. (Cl. 162-181) This invention deals with precipitated finely divided siliceous pigments. It more particularly concerns siliceous paper pigments eifective in enhancing the optical properties of paper.

It has heretofore been appreciated that various properties of paper might be improved by pigmenting and dispersing materials in the paper. Improvement in the optical properties such as opacity and brightness of paper is achieved, for example, by titaniumdioxide pigmentation. Due to the cost of titanium dioxide, however, its use for such purposes is generally limited to the more expensive papers such as writing papers. Other less costly materials such as the natural clays and diatomaceous earths are not efiective pigments for improving optical properties. Thus, while such materials may be incorporated in paper their value in improving optical properties may be regarded as insignificant and they are not accordingly suitable or advantageously used for such purposes.

By virtue of this invention, finely divided, precipitated, flocculated, essentially amorphous siliceous pigments are provided which when dispersed properly in paper serve to enhance strikingly the optical properties of paper. Thus, these special siliceous pigments are of notable effectiveness in providing paper of improved opacity and brightness. Moreover, these pigments are considerably less expensive than titanium dioxide.

The siliceous pigments, it has been discovered, which are so effective in paper are finely divided, white, inorganic, precipitated, flocculated, essentially amorphous siliceous pigments having an average ultimate particle size less than about 0.25 micron, usually between 0.01 and 0.2 micron. Notwithstanding their extremely small ultimate particle size which would indicate poor paper pigmenting qualities, these pigments are eifective by virtue of their state of flocculation. These siliceous pigments are comprised principally of fiocs formed hour a plurality of ultimate particles and have at least 55 percent, preferably at least 75 percent, by their weight in the form of fiocs less than 7 microns, the preponderant weight proportion of such size flocs being in the range of 0.5 to 7 microns.

The extent and nature of the flocculation in these pigments is most important to their paper pigmenting value. Finely divided siliceous materials of less than 0.25 micron, but not properly fiocculated, are by comparison inefiective paper pigments.

Flocs which characterize these siliceous pigments are comprised of a multiplicity of very fine ultimate particles agglomerated or otherwise bound together. Under the electron microscope or other high magnification, these fiocs bear a visual resemblance to grape clusters, the individual grapes representing the ultimate particle of siliceous material. These fiocs are also usually quite porous.

Many siliceous materials includefiocs which may be degraded or, broken up by strong mechanical grinding. On the other hand, in the preferred pigments, the fiocs less than 7 microns in size have their component ultimate particles agglomerated or united with such strength that they resist substantially detrimental deterioration or degradation during those conditions'prevailing during the formation of paper and to which the pigment may be subjected. Thus, these pigments are dispersed in paper principally in the form of docs less than 7 microns in size.

Finely divided, precipitated, white, essentially amorphous, inorganic siliceous materials including silicas and metal silicates such as calcium silicate, aluminum silicate and mixed metal silicates such as calcium aluminum silicate when comprised of fiocs less than 7 microns provide useful paper pigments. The more desirable siliceous materials have an average ultimate particle size of about 0.25 micron or less, preferably about 0.025 to about 0.1 micron, with at least 55 percent by their Weight in the form of fiocs below 7 microns, the preponderant weight proportion of the fiocs less than 7 microns being in th size range of from 0.5 to 7 microns.

Siliceous pigments comprised of such small ultimate particles and having percent or more (up to or even percent) of their weight in the form of fiocs below 7 microns in size are even better paper pigments. When the preponderant Weight portion (at least about 50 percent by Weight) of the docs less than 7 microns in size are in the range of 0.5 to 7 microns, the siliceous pigment is even more effective in paper.

On an anhydrous basis, the siliceous pigments contain at least 50 percent SiO by weight with the balance of their composition being principally one or more metal oxides. Thus, chemically the siliceous materials are silica (a composition containing at least 90 weight percent Si0 on an anhydrous basis) or silicates, e.g., contain Si0 and one or more metal oxides.

Most of these siliceous paper pigments usually are below 200 square meters per gram in surface area. The best silicas and silicates such as calcium silicate normally have surface areas of 10 to 75 square meters per gram. Such surface areas refer to those measured in accordance with the Brunauer-Emmett-Teller method which is described in the Journal of the Americal Chemical Society, volume 60, page 309 (1938).

Two types of water are present in the pigments, bound water and free water. The term free water, as herein employed, denotes water which may be removed lirom the pigment by heating to a temperature of C. for a period of 24 hours in a laboratory oven. Bound water, as used herein, is intended to mean the amount of water which is given ofi from the pigment by heating at ignition temperatures, for example, 1000 C. to 1200 C., until no further Water can be removed, minus the amount of free water in the pigment. Thus, these pigments are referred to as hydrated in that they contain some water, either as bound water and free water. Bound water and free Water usually each constitute 2 to 10 percent by weight of the composition.

One of the siliceous pigments is finely divided, precipitated, hydrated, porous, flocculated, amorphous silica composition having an average ultimate particle size of about 0.1 micron or less, usually in the range of 0.035 to 0.08 micron, and having at least 55 percent by its weight below 7 microns in floc size. The'best silicas generally have surface areas, as precipitated, of 10 t 75 square meters per gram.

The amount of bound water in the silica may vary. Typical silicas have a chemical composition basis their SiO and bound water content represented by the formula:

H O. (SiO 3 wherein x is a whole or fractional number from 3 to 85. and H 0 represents bound water.

Usually, free water constitutes from 2 to 10 percent by weight of thersilica in the pigment.

Silica, as used herein, refers to a pigment containing at least 90 percent SiO by weight on an anhydrous basis.

. I 3 Comprising the balance of the pigment, when the SiO-,, content is less than 100 percent, e.g., say 94 percent SiO by weight on an anhydrous basis, are minor quantities of other oxides. One such oxide is often calcium oxide, present usually inconcentrations ranging normally between 0.5 to 6 weight percent. .Oth'er constituents present mainly in minor concentrations of 0.1 to 2 percent by weight 'on an anhydrous basis include iron and aluminum oxides, sodium chloride and carbon dioxide.

' These silicas also often contain small "quantities of sodium oxide. When present, the sodium content measured asNa O is well below 1.75 percent by weight. The silica also may include minor quantities'of other metals such as alkaline earth metals, such as magnesium,-barium and zinc. These metals are present probably as a metal oxide,'possibly in chemical association with the silicas. Suitable paper pigments alsof include 'finelydivided, flocculated, inorganic metal silicates having flocs and ultimate particles of the desired size. Whether'these metal silicas arechemically combined mixtures of the metal oxide (or metal oxides) and SiO;; is not-altogether clear. Nevertheless, these siliceous compositions ana- 2,94s,97o I Q r "'a continuous countercurrent decantation system. Slurry was pumped to this system at the rate of 3 gallons perminute and about 90 percent of the alkali was removed.

in the first two Dorr tanks with a counterflow of about 15 gallons per minute of icondeusate at about 93 C.

' Continuously removed frorn this-system, the slurry was acidified batch-wise with hydrochloric acid to about 4.5

pH. This acidified slurry was returned to a third Dorr tank-to remove residual :sodium chloride to a concentrafrom .15 to 18 percent in the hydrochloric acid Wash.

lytieally conform (on an Si0 and metal oxide or oxides.

basis) to the equation:

MO.(SiO 7 I wherein M represents one or more metal cations such EXAMPLE I Seventeen thousand gallons of sodium silicate containing a maximum of 0.08 percent NaCl by weight, Na O (SiO diluted to a concentration of 40.6 grams Na O per liter, was heated with direct steam to 75 C., and charged into a tank 26 feet tall and 18' feet in diameter to establish a liquid depth of 8 feet. Gas containing about 10.5 percent by-weight carbon dioxide and as one or more of the following: calcium, magnesium,

After washing, the pigment was filtered on acontinuous rotary vacuum, filter wheel at filtration rates of from 10 pounds per hour per square foot to 50 ;pounds per hour per square foot. After such filtration, the :cake was driedin a steam tube-heatedrotary dryer using 125 pounds per square inch gauge steam. After drying to 2 to 5 percent by weight free moisture, the material was milled in an 18 inch Raymond vertical mill.

Silica prepared in this manner had a surface area of 41 square meters per gram, an ultimate particle si'ze diameter as determined under the electron microscope of from 0.09 to 0.1.1 micron (about 0.1 micron) and 69 percent by weight of flocs of a diameter less than 7 microns, as determined by the afterdescribed sedimentation test, The predominant .weight proportion of the flocs sized less than 7 ,microns was in the range of 0.5

to 7 microns, t r l 1 Following the above general procedure, but varying the concentration ,of sodium silicate, the rate of carbonation and temperature makes it possible to obtain finely weightas the flocs below. 7 microns in size.

the balance mainly nitrogen was bubbled into the silicate solution through a6 foot diameter dispersion ring concentrically disposed in the tank '4 feet above the tanks bottom and downwardly facing, one-inch diameter holes spaced one foot part. Agitation was supplied by rotating a 45 revolutions per minute arc-blade turbo agitator with a 6 foot diameter blade located approximately 2 feet below the liquid level 'in the tank. a

=The gas wasfed at the rate of about '1400 cubic feet per minute (standard conditions, 760 millimeters mercury pressure at 0 C.) for the first 3,hours of carbonation, after which the rate was reduced to about 1300 cubic feet per minute at standard conditions for an additional 13 hours. After about 2 hours and 26 minutes, silica precipitation occurred, and after 6 and /2 hours. of carbonation, 100 percent carbonation was reached as determined by the double end point method of titration for soda ash-bicarbonate solution. The final degree of carbonation: was 141 percent.

At the end of carbonation, live steam was introduced into the bottom of the slurryat a'r'ate such that the boilpoint wasreached in about 30 minutes and this boil-, ing was continued for :minufes." Suflicient Arquad 2C, a commercial dialkyl dimethyl ammonium. chloride, was then added to establish 0.3 percent by weight concentrationthereof, on a drysolids basis.

This slurry was washedin a three Dorr tank system,

divided fiocculated silica of the desired quality having surface areas ranging usually from about .15 to 90 square meters per gram, an average ultimate particle size ranging as low as 0.025 micron and at least percent byits The efiectivenss of pigmenting paper with silica produced according to Example I is shown in these examples:

' EXAMPLE II A pulp beater was charged with 360 grams of bleached kraft pulp dispersed in 23 liters of water and beaten in a Niagara heater for about 70 minutes to approximately 400 milliliters freeness (Canadian standard). A 21.1 kilogram portion of this beaten pulp was transferred to a rubberized drum and further mixed with an Easte mixer. 7

To l8.1 kilograms of this pulp, a 400 cubic centimeter aqueous slurry containing 28 grams of silica prepared in accordance with Example :1 and having asurface area of 41 square meters per gram, a bulk density of 0.32 gram per cubic centimeter at 3 pounds per square inch gauge and an average ultimate particle size under the electron microscope of 0.09 to 0.11 micron was then mixed with the above pulp slurry for .5 minutes; Sixty-nine percent i by weight of the flocs. of this pigment were below 7 miwith a black backing to the brightness of the same sheet with a white backing), a .brightness of 82.7 percent Hunter, a tear factor of 41 grams per gram sheet and a burst factor of 9.2 pounds per square'inch perone gram sheet. The paper hadan ash of 8.1 percent indicating excellent pigment retention. Paper of this opacity, brightness, and tearand burst factor possesses an excellent combination of desirable qualities. 7; f a V 7 EXAMPLE III Paper was produced according to the procedure of Example II, employing a silica prepared by the procedure of Example I andhaving a surface area of 32 square meters per gram and a bulk-density of 0.46 gram per cubic centimeter at 3 pounds per square inch gauge. The weight percent of flocs below 7 microns in diameter for this silica pigment was 63.9. In this experiment, suflicient aluminum sulfate octadecahydrate by weight of the pulp on a dry basis was added at each point and the chest water pH was 5.2, while the white water pH was 5.3. This paper had an opacity of 81.4, a brightness of 84.3 percent Hunter, a tear factor of 41 grams per gram sheet, and a burst factor of 9.4 pounds per square inch per one gram sheet. It had an ash of 8.5 percent by weight, demonstrating excellent pigment retention. Again, the above data indicates the preparation of excellent quality paper.

EXAMPLE -IV In this example, silica prepared as in Example I was employed except that it was acid treated prior to heat conditioning. Thus, the silica'had a surface area of over 124 square meters per gram, although upon precipitation in an alkaline slurry, its surface area was 28 square meters per gram. The silica had a bulk density of 0.38 gram per cubic centimeter at 3 pounds per square inch gauge, an ultimate particle size of 0.09 micron in diameter under the electron microscope and 55.9 percent by weight of its flocs were less than 7 microns in diameter.

Paper pigmented with this silica by the procedure of Example II had an ash of 7.8 weight percent, an opacity of 80.0, a brightness of 82.5 percent Hunter, a tear factor of 44 grams per gram sheet and a burst factor of 9.0 pounds per square inch per one gram sheet.

EXAMPLE V Using a 72 liter batch, and carbonating with 100 percent pure carbon dioxide for 4 hours (equivalent to 150 percent theoretical carbonation), at 85 C., the pigment was prepared from a sodium silicate solution of Na (SiO containing 20.3 grams Na O per liter and 10 grams sodium chloride per liter. Thereafter, it was micronized. This silica pigment had a surface area of 30 square meters per gram and 98 percent by its weight as flocs less than 7 microns in diameter.

EXAMPLE VII The above pigments were then used to pigment paper according to the paper making operation as described in Example II above, and the paper produced thereby tested for the significant paper qualities with the following results:

Paper Properties Pi ent Ash Bright;-

Weight Opacity ness Tear 1 Burst 3 Percent Example V I s. 4 81.3 83.2 37 s. 2 Example VI 8. 4 83. 5 81. 4 39 8. 7

1 Grams per one gram sheet (8 inch by 8 inch square). Pounds per square inch per one gram sheet.

' This example illustrates a typical method of preparing an efiective calcium silicate or like alkaline earth metal silicate paper pigment;

EXAMPLE VIII within the tank at a point representing the 60 gallon level thereof, and provided with an external leg to control the liquid contents of the tank at between 60 to gallons. Calcium chloride solution Was introduced via a one-half inch line into the bottom of the tank. Air was passed into the bottom of the reactor, imparting a gentle rolling agitation to the slurry.

Aqueous solutions of the respective reagents containing 100 grams per liter of calcium chloride and 100 grams per liter of SiO were fed to the reactor through their respective feed lines at average feed rates of 2.95 gallons per minute of silicate solution and 198 gallons per minute of calcium chloride solution. This maintained an excess calcium chloride level of approximately 15 to 20 percent. In this manner, slurry was precipitated at the rate of about 5 gallons per minute and contained approximately 7.5 percent solids by weight.

The precipitated slurry was withdrawn at a rate corresponding to the rate of its formation and pumped directly to a storage tank and thereafter washed in a three Dorr tank system using an 8 to 3 water-slurry ratio. At a feed rate of 3 gallons of slurry per minute, the final wash slurry contained about 8 percent solids by weight. This washed slurry was filtered on an Oliver wheel and the resulting filter cake containing an average of 23.6 percent solids by weight was dried in a Louisville dryer. After drying, the pigment was milled in a vertical mill arrangement to provide a product containing less than 0.1 percent residue on a 325 mesh screen.

Calcium silicate produced and recovered in the above manner had a surface area ranging between 32 and 36 square meters per gram, a pH of 10.5 in aqueous slurry and contained about 0.47 percent NaCl by weight. As precipitated and removed from the reactor, the surface area was about 44 square meters per gram, the aging apparently being responsible for a surface area decrease of 8 to 10 square meters per gram. Bound water in the concentration of about 10 percent by weight of the pigment and free water in the range of 3.3 to 5.3 percent were present. The calcium silicate had a composition conforming to the formula CaO(SlOz)3 1 4 75. An average composition for the product was CaO(SiO By following the above procedure but varying the amount of excess calcium chloride employed, calcium silicate pigments having final surface areas other than 32 to 36 square meters per gram but in the range of 10 to 50 square meters per gram were prepared. Thus, using calcium chloride in proportion of from 0 to 1 or 2 percent stoichiometric excess provided calcium silicate having a surface area in the range of 10 to 16 square meters per gram. With calcium chloride excesses of from about 4 to 10 percent, calcium silicate pigments resulted having surface areas in the general range of from 20 to 40 square meters per gram. Excess calcium chloride concentrations ranging from about 15 to 20 percent gave pigments having surface areas ranging between 35 and about 50 square meters per gram.

Following the drying, the calcium silicate (or like alkaline earth metal silicate, such as magnesium silicate or strontium silicate) is ground and air classified to separate a coarse fraction and produce an alkaline earth metal silicate in which at least 55 percent, and preferably 70 to percent by weight of the flocs are below 7 microns, the preponderant weight thereof being 0.5 to

V in which 86 percent by weight of the fiocs werebelow 7 microns and 'a'fraction in whichbut 3 9' percent by weight of the docs were below 7m icrons; The fractions werethen used to prepare paper according to the pro-' cedure of Example II with these results: Y

Paper Properties Weight Percent Flees Less Than 7 Mlcrons Ash, Weight Percent Bright- 7 .Opacity ness 6.2 2 A 77.3 6. e a 79.8

Calcium silicate as precipitated in Example VIII may be reacted with an aluminum salt' such as aluminum sulfate and the resulting pigment dried and classified, if necessary to achievea product having proper. floc size. Alternatively, appropriately flocculated calcium silicate maybe rea'cted with "aluminum sulfate or the like in a pulp suspension to provide an alumina-silica pigment as shown in Example X.

EXAMPLE X The pulp heater is charged with 360 grams of bleached kraft pulp, dispersed in 23 liters of water and beaten in a Niagara heater for about 70 minutes to approximately 400 milliliters freeness-(Canadian standard). A 21.1 kilogram portionof this beaten-pulpistransferred to a rubberized drum and further -m'ixe'd with an Eastern mixer. To 18.1 kilograms of this mixed pulp, 400 cubic centimeters ofjaqueous aluminum sulfate solution containing 19.6 grams of'Al (SO .18I-I O (7.0 percent by weight of the pulp) is added and mixed therewith for 5 minutes. 1 V q A 400 cubie'centimeter' aqueous slurry containing 28 grams of calcium silicate having a surface area of 31 square meters per gram and prepared as described in Example VIII, an average ultimate particle 'size from 0.04 to 0.08 micron and a packed bulk density of 0.377 gram per milliliter at 3 pounds per square inch gauge is then mixed with the above pulp slurry for an additional 5 minutes. Prior to use, this pigment Was air classified to remove coarse particles and to provide a product in which the aggregates were preponderantly" in the range of l to 7 microns. A small' additional quantity of A1 (SO .l8H O (0.7 percent by weight-of the pulp on a dry basis) was added to adjust the slurry and white Water to pH 5. This provided a slurry containing percent calcium silicate and 7.7 percent Al (SO .18H O based on the dry weight of the pulp. This pulp slurry is then sheeted on a laboratory Noble-Wood sheeting machine.

Paper produced in accordance with the above procedure and having an alumina-silica pigment dispersedtherein had an opacity of 80.6 (the ratio of brightness of a single sheet with a black backing to the brightness of the same sheet with a white backing), a"brightness of'81.2, a tear factor of 36 grams per gram of sheet and a burst factor.

of 10.1 pounds per square'inch per gram of sheet; Such paper had an ash of7 .2 percent by weight indicating good pigment retention. Paper having high opacity and brightness of this character is of highquality. The tear and burst factors indicate commendable strength for pigmented paper. A 1

-Such aluminum silicates are finely divided, soft, pulverulent; amorphous, flocculated products of the composition A1 O .(SiO where y i5 6 to about 100, including fraction numbers. Their ultimate particle size is below about 0.2 micron, often below 0.1 micron and above about 0.05 micron. The silicates may contain substantial of'metals other than aluminum. Thus, a pigment having the following composition is illustrative:

Ignition loss at 1000 C p V 16 7 Loss on drying at 105 C. SA S a f 7 46,1 R 0 (almost entirely A1 0 10.88 F 203 V V 1 a C Q g 11.82 Ms 8 N 2 7 e 10.04 Chloride 7 I a 0.04 3 3-3. 2 6.6

As shown above, the alumina-silica products may be,

and frequently are, mixed with other salts such as calcium sulfate and the like. Quite often, they also contain other metals such as calcium, zinc, barium, and the like, the presence of which are advantageous in order to maintain the basicity' of the pigment. I 'They are invariably amorphous. V e p The products normally contain 2 to 10 percent by weight of free water (waterwhich can be driven on by heating at 105 C. for 24 hours) and 2 to 10 percentby weight of bound'water. 1 7

It is also possible, by the method of precipitation, to produce alumina silica pigments having their lines predominantly -less than 7'microns. Thus, by recourse to specific techniques an aluminum salt such as aluminum sulfate may be reacted with finely divided calcium silicate. to produce utilimately'an alumina-silica composition so flocculated. One such method entails partially reacting a slurry of calcium silicate with aluminum sulfate and recovering the product. This product may be thereafter further reacted with aluminum salt, preferably in a pulp slurry. As illustrated by the immediately ensuing examples, proper performance of such method provides a flocculated alumina-silica pigment of appropriate floc size.

EXAMPLE XI An aqueous slurry of calcium silicate produced by a method in which solutions of sodium silicate and calcium chloride were mixed under conditions of violent agitation provided by the centrifugal action of the pump was di vided up into batches containing 400 grams of calcium silicate (basis calcium silicate dried at 105 C.). The slurry containing 5.4 weight percent calcium silicate (basis dry weight), 48 grams per liter of sodium chloride and 34 grams per liter of calcium chloride. Calcium silicate so prepared contained 325 moles of Si0 per mole of 'CaO and had a surface area of 96 square metersj per gram. When recovered from the slurry and dried, the calcium silicate was a white, finely divided, precipitated, amorphous material having an ultimate particle size of less than 1.0 micron and more particularly from 0.0110 0.05,

averaging 0.03 micron, so flocculated that the fiocs were Such calcium silicate has a typical composition:

Weight percent:

A 400 gram sample of this calcium silicate requires,

(determined by experimentation) approximately 640 milli liters of an aqueous solution of alum containing 500 grams Weight percent per liter of alum as Al (SO .1811 for complete neutraliz'ation (extract essentially all of the calcium oxide).

Difierent quantities of an aqueous alum solution (containing 500 grams per liter of Al (SO .l8H O) were added to individual batches of the calcium silicate slurry at about 25 C. according to the data tabulated in the following table:

Volume of Amount of Alum Added Alum Batch Solution Equivalent pH of BE'I.

Added to Percent Slurry m. /gm. (milliliters) Neutralization A (Control) 0 0 B 64 9. 9 8. 123 224 34. 7 8. 3 169 320 49. 5 8. 2 180 416 64. 3 7. 7 239 576 89. 0 6. 2 127 640 99. 0 4. 7 130 1 Surface area, square meters Emmett-Teller method.

Slurries of alumina-silica compositions so produced were filtered, washed, oven dried at 105 C. and micropulverized. Samples of batches D and E had a composition of:

per gram, as measured by the Brunauer' The pH referes to the pH of the slurry produced by reslurrying in water the washed, dried composition. Weight percent loss at 105 C. is the free water in the pigment as measured by loss in weight due to removal of water by drying at 105 C. for 24 hours. Weight percent loss on ignition represents the amount of water lost while heating the pigment at 1000 C. Ignition loss measures the bound water and free water found in the pigment, bound water being the difference between the ignition loss and loss of weight by heating at 105 C. Ap parently, the bound water is chemically combined in the pigment and not readily removed except by rather severe heat treatment.

Samples of the alumina-silica pigments produced (batches A through G inclusive) were studied under an optical microscope and by use of photomicrographs. Sample A (the control) was comprised mainly of large aggregates measuring from 30 to 60 microns in diameter. Photomicrographs of sample B indicated it contained fewer aggregates measuring from 30 to 60 microns in diameter than sample A. The number of aggregates in the 30 to 60 micron diameter range decreases in pigments prepared with greater amounts of alum. Thus, sample C was comprised of considerably less of such large ag gregates than sample B.

This decrease in large aggregates as the increased degree of neutralization increased was observed with those products prepared using less than 50 percent of the alum required for complete neutralization. Products prepared using from about 40 or 50 percent and 65 percent of the alum required for complete neutralization were practically free of aggregates. Photomicrographs of samples D and E indicated practical absence of 30 to 60 micron flocs as well as any flocs of discernible size.

Alumina-silica (or aluminum silicates) prepared with more than about 65 percent of alum required for full neutralization were comprised of increasing amounts of ag-." gregates considerably larger than 7 microns;

However, by partially reacting 'or neutralizing calcium silicate (to the extent of to 70 percent of complete neutralization) with aluminum sulfate or like aluminum salt in a calcium silicate slurry such as is provided by reaction of sodium silicate and calcium chloride, a pigmentary calcium silicate-alumina-silica composition is obtained which upon recovery, drying and mechanical pulverizing is admirably suited for enhancing the optical properties (opacity and brightness) of paper. Paper of enhanced optical properties is provided by including the partially neutralized calcium silicate composition in the pulp slurry along with aluminum sulfate and sheeting the slurry into paper. Reaction between the composition and aluminum sulfate occurs with the result that an aluminasilica (aluminum silicate) composition comprised in thej main of flocs sized between 0.5 and 7 microns is dispersed in the sheet. superior to those of papers prepared by including unneutralized calcium silicate and alum in pulp slurries prior to sheeting.

The following example illustrates the use of silicates (provided by neutralizing 30 to 70 percent of a calcium silicate with an aluminum salt) comprised of flocs in the main sized from 0.5 to 7 microns for improving the optical properties of paper.

EXAMPLE XH A pulp beater was charged with 500 grams of bleached kraft paper dispersed in 23 liters of water and beaten in a Niagara beater for about minutes to approximately 400 millimeters freeness (Canadian standard). A 7.0 kilogram portion of this beaten pulp was transferred to a rubberized drum and further mixed with an Eastern mixer.

A 300 cubic centimeter portion of an aqueous slurry containing 14 grams of a pigment produced by the process described in Example XI above was then mixed with the above plup slurry for an additional 5 minutes. Suificient Al (SO .18I-I O was added to the slurry to adjust the slurry in white water to about pH 5. Depending on the particular pigment and its degree of neutralization, according to the process described in Example XI, this quantity varied but was sufficient to further neutralize the pigment.

Thus, a slurry containing 10 percent of the sample of calcium-aluminum silicate basis the dry weight of the pulp was provided. This pulp slurry was then sheeted on a laboratory Noble-Wood sheeting machine.

A series of runs were performed in this fashion with the different pigments and the properties of the sheets so prepared obtained as follows:

Percent Paper Properties :Calcium Pigment silicate sample Neutral- Ash, Brightized Weight Opacity ness Tear 1 Burst Percent 1 Tear factorgrams per gram of sheet 8 inches by 8 inches. 8' 131111813 factor-pounds per square inch per gram of sheet 8 inches by me es.

' Analysis of the pigment content of the sheeted paper Optical properties of such paper are mm sulfate in a pulp slurry and paper sheeted the resultingslurry according to the procedure of Example II with these results:

Percent BET, Paper Properties 7 Calcium Square Pigment Slurry, Silicate Meters Sample pH Neutral- Per Ash Bnghtized Gram Weight Opacity ness Tear Burst .1 percent pigmefifllthe bpaeiry is but am, pacer and, with 90 percent-neutralization, the opacity is but 73.5. .Siliproportion of their-flocs less -than'7. microns as shown by Exammemu. I I

An aqueous slurry ofcaliumsilieate was prepared by reacting with gentle agitation aqueous. solutions ofsodium silicate 'and calciuin chloride. This slurry contained 8.72 weight percent of calcium silicate (dry weight basis of Lcalcium-. silicate), .4. 02 'gnams per liter of calcium chloride, 54.6.3. .grams per liter ofsodium chloride, and The calcium silie.

wases's'entially :free of sodium silicate. cate had thefollowing analysis:

A plurality of equal batches of this slurry were. provided, each containing 8 00 grams of dry solids, Each batch was stirred in a bucket and an amount of alumi num chloride (as AlCl .6H O) in aqueous solution sufficient .to provide the indicated degree of partial. neutralization was added. After 5 minutes of stirring, the pH was obtained, and half the slurrywas filtered, the filter.

cake washed with Water, dried in an oven at 105 C. and milled by passage through a micropulverizer.

Pigment samples 1 to 4 were flocculated, finely vided products having over 55 weight percent 'otitheir fiocs below 7 microns.

" Alumina-silica pigments having at least 55 weight per 7 cent of their flocs less than 7 microns are alsoprepared by simultaneously mixing with intense agitation an aluminum salt such as aluminum chloride, calcium chloride or like alkaline earth metal salt and alkali metal silicate, e.g., sodium silicate. This intense agitation i s provided, for example, by feeding the respective re: actants to a'centrifugal pump and accomplishing the mixing by the agitation developed in the pump chamber as illustrated by the following examples:

EXAMPLE XIV A streamof an aqueous solution, of sodiumsilicate containing grams per liter of Na O(SiO and a stream of an aqueous solution containing 50 grains per liter of a mixture of calcium chloride and aluminum chloride (the mole ratio of calcium chloride to aluminiim chloride being 1 to 1) were introduced into a centrifugal pump as described above, 'proportioning the streams .so' that the metal chlorides were 10 percent in excess of stoichiometric. The slurry was filtered on ;a Biiehner funnel and the filter cake dried at about l00 and pulverized. The resulting product contained 68.64'percent SiO 7.31 percent A1 0 and 8.01 percent CaO'.

EXAMPLE XV I Pigmentary, flocculate'd calcium silica were precipitated by feeding an aqueous sjtre' V 2 Q2)3.28 o n ng 4. grams pe liter o N a d all e l t m c t i g grams P of calcium chloride and varying concentrations of num chloride to a centrifugal pump'while' proportionjing r the relative solution feed, rates and relative concentrations of aluminum chloride and calcium chloride in ac.-

A plurahty of pigments were then reacted with alurmcordance w th the conditions hsted in Table 1 below.)

Table I V Grams per Liter in A1613 1 1.Percent Chloride Stre m Equivalent Temp Stoichio- Surface Run N o. r p to Percent 6. metric Ex- Area, j V p of Sojchiocess'of in /g A1 9 .A'lQ a-fiHzQ met i 9 0 1 1 as 95 25 13s 3191 2 40 25 *llO 348 20 43 f 3 .331 4 12.5 Y 22 25 6 2 115 5 28.8 47 5 82 127 6 43 .15;: 7 v6.5.. 133 a 19.3 27 5 25. 7 r '99 9 a 28.7 V '39 50- 47 124' s 10 28.7 Q 48 75 1175, '11 i -52- 52 92 200 a 12 I 53 54 -2 95' 1-: .53 50 2 I 95;, {1'91 ',14(A) 1 '53 '45 2 '85 4 103 15 3 28.7 48. 75 as; use

i I (AYPigment precipitated while recirculatingslurry to pump. 7

p, (-B) Slurry boiled for on'e'hour. V I v Percent by Weight S l g Ignlti amp e oru on Run N 0. Loss (Bound SIO: A110 CaO N8 C1 and Free Water) 16. 8 65. 4 6. 67 9. 48 0. 53 0. 02 18. 7 62. 4 5. 76 9. 85 l. 80 2. 49 17. 7 64. 5 6. 61 8. 99 1. l3 1. 25 17. 8 64. 6. 79 10. 17 0. 70 0. ll

EXAMPLE XVI Calcium aluminum silicates prepared according to Example XV were used to pigment paper. This was done by charging to a Niagara beater 500 grams of bleached kraft pulp dispersed in 23 liters of water and beating for about 95 minutes to approximately 400 milliliters freeness (Canadian standard). A 7.0 kilogram portion of this beaten pulp was'transferred to a rubbero ized drum and further mixed with an Eastern mixer.

A 300 cubic centimeter portion of an aqueous slurry containing 14 grams of a calcium aluminum silicate slurry produced as described in Example XV was mixed with the prepared pulp slurry for minutes. Suificient Al (SO .18H O was added to adjust the slurry and white water to about pH 5.

pulp slurry was sheeted on a laboratory Noble- Wood sheeting machine and representative sheets of the respective runs were tested to evaluate their brightness, opacity, tear, burst factor and ash contents with these The above data demonstrate the highly efiective contribution to optical properties of paper provided by use of flocculated, finely divided calcium aluminum silicates having in excess of 55 percent by weight of its flocs below 7 microns.

These siliceous pigments are also used with extreme efiectiveness in combination with paper grade titanium dioxide for pigmentation of paper. Especially unexpected enhancement of opacity, for example, is realized using mixtures containing between 35 and 95 percent titanium dioxide by weight of the titanium dioxide and siliceous pigment as shown by the following data obtained by forming paper as in Example 11, using mixtures of titanium dioxide and a properly flocculated calcium silicate.

Percent Pigment Paper Properties Ash, Bright- Sllicate T107 Weight Opacity ness Percent The floc sizes herein mentioned are determined by a specific procedure, which although not necessarily providing an absolute measurement, does establish a scientifically significant, readily determinable standard. The procedure involves warming an aqueous pyrophosphate solution containing 0.05 percent pyrophosphate by weight to 30 C. in a Water bath. This solution is prepared by neutralizing distilled water to a pH of 7 with sodium hydroxide and dissolving 0.84 gram of'tetrasodium pyrophosphate decahydrate in each liter of neutral water. To approximately 250 cubic centimeters of this heated solution is added 2.50 grams of silica and the resulting mixture is treated in a Waring Blendor for exactly 15 seconds, Whereafter the slurry is transferred to a 250 milliliter graduated, stoppered cylinder and mixed thoroughly by inverting the cylinder. The level is adjusted to the 250 milliliter mark before mixing.

After mixing, the cylinder is placed in the upright position and a timer started. Immediately, a 5 milliliter pipette so fittedwith a rubber stopper that it will reach exactly 2.2 centimeters below the 245 milliliter mark of the cylinder is inserted and a 5 milliliter slurry sample is withdrawn into a tared weighing bottle. Afiter 49 minutes, a second sample is withdrawn into a tared weighing bottle and the respective samples are dried at C. in an oven overnight.

'The ratio times one hundred of the Weight of the second sample divided by the weight of the first sample is the percent of flocs less than 7 microns in diameter.

These exemplary siliceous paper pigments are useful for preparing paper by procedures which include incorporating the pigment in the paper pulp slurry and thereafter forming the pulp into paper by recognized sheeting techniques. Thus, the desirable siliceous pigment may be simply added to a pulp slurry and well dispersed therein pnior to sheeting in the conventional manner the slurry.

In one Well suited procedure, the siliceous pigment is added to the pulp slurry after it has been formed or after most of the beating is accomplished. This avoids subjecting the pigment to unnecessary agitation or beating and minimizes the opportunity for disrupting the desired fiocculated condition of the pigment. After the pigment is included in the pulp slurry, it is Well distributed therein by agitation, preferably mild agitation.

Pigment is incorporated in the slurry usually in an amount of from 0.1 to 5 to 10 percent by weight of the pulp basis the dry pulp weight. Even larger amounts of pigment are used, although primarily for the preparation of special types of papers.

Most of these pigmentary materials are of further value and provide excellent pigmented paper because they are Well retained in the sheeted product. Upwards of 70 percent and often essentially all of the pigment incorporated in the aqueous pulp slurry is retained in the paper sheeted from the slurry. Pigmented papers contain from 0.1 to 10 percent siliceous pigment by'weight of the paper as sheeted and dried, and exclusive of any coating that may be applied to the paper. More heavily pigmented papers, e.g., papers containing upwards of 10 percent and up to 30 percent siliceous pigment by weight, also may be provided.

The pigment is well dispersed in the sheeted paper. I *It may be regarded as distributed uniformly or essentially uniformly throughout the sheet in admixture with the other components, notably the cellulosic pulp.

Sheeted paper is usually regarded as a fibrous matenial comprised of non-oriented cellulosic pulp fibers. The pigment is well distributed between and around these individual'fibers. 7

Paper comprised of all pulps susceptible generally regarded asqsuitable for sheeted paper are useful here. Thus, chemically treated pulp typified by sulfite, soda, or kraft pulp, semi-chemical pulp, or mechanically ground plup, or mixesthereof may comprise thepulp component. Besides virgin pulp, broke, i.e., cuttings and scraps of paper recovered from paper making operations, may constitutea portion of the cellulosic pulp.

Other paper manufacturing operations normally .associated with the preparation of sheetedpape-r may also be'practiced. in conjunction withthe use of these pigments in paper. Thus,; the pigments are usefulin sized or unsized paper. For sized paper, a rosin size such as a soluble alkali metal rosin soap, .e.g sodium .resinate or other typical size is included in the pulp slurry and precipitated concurrently qr prior to the addition of the pigment to the'slurry by trivalentv or tetravalent metals such as titanic sulfate or aluminum sulfate. Coloring matter may likewise beincluded during the feeding cycle.

The pulp may also be bleached by the addition of chlorine in. an oxidizing form orby other recognized paper bleaching expedients. Generally, bleaching is accomplished prior to including the pigmentary materials in the pulp, although this is not essential.

In accordance with accepted paper vmanufacturing techniques, the pulp slurry in which the siliceous pigment is incorporated is sheeted into paper on a wire of a Fourdrinier or other paper forming apparatus, Subsequently, the paper may becalendered, dried, and if desired, coated in accordance with standard paper procedures. 7 7

It is desirable in preparing these pigmented papers that the pulp slurry ,be acidic, ,e.g., at pH ,4 to 7, notably about pH 5, when sheeted. Acidifying agents such as aluminum sulfate may be included in the pulp slurryrto render it suficiently acidic. Other such materials as inorganic acids, notably the mineral acids, inorganic acid salts and the like also are acidifying agents, If the siliceous material includes a metal oxide or other impurities which byrion-exchange or other chemical interreactions react with aluminum sulfate or other acidifying agent, the chemicalcomposition of the pigment may be somewhat altered. For example, the calcium oxide content of calcium silicate may be partially or completely reacted with the aluminum sulfate such that in the final paper it will be'present in lesser concentrations basis the weight of the pigment dispersed in the paper. The aluminum sulfate will replace the calcium oxide in part or in whole and the pigment within the paper will be an alumina-silica composition. a

This application is a continuation-in-part of application Serial No. 616,595, filed October 18, 1956, which application is a continuation-in-part of applications Serial No. 770,169, filed August 22, 1947; Serial No. 277,760, filed March 21, 1952, now abandoned; Serial No. 283,721; filed April 22, 1952, now United States Letters Patent 2,805,955; Serial No. 355,865, filed May 18, 1953, now United States Letters Patent 2,786,777; Serial 'No. 473,057, filed December 3, 1954, now United States Letters Patent 2,786,776; and is a cont-inuation-in-part of application Serial No. 646,224, filed March 15, 1957, which application is a continuation-in-part of applications Serial No. 616,595, filed October 18, 1956; Serial No. 471,533, filed November 26, 1954, now'United States Letters Patent 2,837,495; and Serial No. 473,057, filed D'ecember 3, 1954 The present invention has been described with specific reference to certain details, however, it will be understood'i'that it is not intended that the invention be constru ed as limited to such details except insofa'r they resembling grape clusters, the individual grapes of which are. representative of ultimate particles of siliceous material, the average size of the ultimate particles being less than 0.25 micron, at least 55 percent by weight of the pigment flocs being below 7 microns. i

2. Finely divided, precipitated, flocculated siliceous pigment, the pigment being in the form of flocs of ultimate particles,;said flocs under high. magnification visually resembling grape clusters, the individual grapes of which are representative of ultimate particles of siliceous mat r the v r g s ze e ltim Particle be n ess than. :25 micron, at. l as .15 percent by weight. of the pigment flocs being below 7 microns.

3. Finely divided, pnecipitated, flocculated siliceous pigment, the pigment'being .inthe form of flocs of ultimate particles, said flocs under high magnification visually resembling -grape-clus'ters, the individual grapes ofwhich are representative ofultim'ate; particles of siliceous material,--the average size 'of the' ultimate particles-being between 0.025 and 0.25 micron, -at least percent by weight 9f the-pigmentflp'cs beingbelow-7 microns.

4.;Finely divided, precipitated, flocculated siliceous pigment, the pigment being-inihe form :of flocs of ultimateparticles, said flocs under high magnification visually resemblingg'rape clusters,the individual-grapes ofwhich ar'e-representative of ultimate particles of. siliceous' material, the average size of the ultimate particles-being less than 0.25 micron, at least 55 percent by weight of the pigment flocs being below 7 microns and the preponderant weight of these flocs being in the range of 0.5 to 7 microns in size.

5. Finely divided, precipitated, flocculated siliceous pigment, the pigment being in the form of flocs of ultimate particles, said flocs under high magnification visually resembling grape clusters, the individual grapes of which are representative. of ultimate particles of siliceous material, the average sizeof the ultimate particlesbeing between 0.025 and 0.25 micron, at least 55 percentby weight of the pigment flocs being below 7 microns and the preponderantweight of these flocs being in the t l age of 0.5 to 7 microns in size.

6. Paper having dispersed therein finely divided, preindividual grapes of which are representative of ultimate particles of siliceous material, the average size of'the ultimate particles being between 0.025 and 0.25-micron, at least 55 percent by weight of the pigment flocs being below 7 microns. a

"8. Paper having dispersed therein finelydivided, precipitated flocculated siliceous pigment, the pigment being in the form of fiocs of ultimate -particles, said flocs under high magnification visually resembling grape clusters, the individual grapes of which are representative of ultimate particles of siliceous material, the average size of-the ultimate particles being less-than 0.25 micron, .at least 75 percent by weight of the pigment flocs being below-7 m crons.

9. Paper having dispersed therein finely divided, precipitated, fiocculated siliceous pigment, the pigment being in the form of fiocs of ultimate particles, said fiocs under high magnification visually resembling grape clusters, the individual grapes of which are representative of ultimate particles of siliceous material, the average size of the ultimate particles being between 0.025 and 0.25 micron, at least 75 percent by weight of the pigment fiocs being below 7 microns.

l0. Finely divided precipitated fiocculated siliceous pigment, the pigment being in the form of fiocs of ultimate particles, said fiocs under high magnification visually resembling grape clusters, the individual grapes of which are representative of ultimate particles of siliceous material, the average size of the ultimate particles being less than 0.25 micron, at least 75 percent by weight of the pigment fiocs being below 7 microns and the preponderant weight of these fiocs being in the range of 0.5 to 7 microns in size.

l1. Finely divided precipitated fiocculated siliceous pigment, the pigment being in the form of fiocs of ultimate particles, said fiocs under high magnification visually resembling grape clusters, the individual grapes of which are representative of ultimate particles of siliceous material, the average size of the ultimate particles being between 0.025 and 0.25 micron, at least 75 percent by weight of the pigment fiocs being below 7 microns and the preponderant weight of these fiocs being in the range of 0.5 to 7 microns in size.

12. Paper having dispersed therein finely divided precipitated fiocculated siliceous pigment, the pigment being in the form of fiocs of ultimate particles, said fiocs under high magnification visually resembling grape clusters, the individual grapes of which are representative of ultimate particles of siliceous material, the average size of the ultimate particles being less than 0.25 micron, at least 75 percent by weight of the pigment fiocs being below 7 microns and the preponderant weight of these fiocs being in the range of 0.5 to 7 microns in size.

13. Paper having dispersed therein finely divided precipitated fiocculated siliceous pigment, the pigment being in the form of fiocs of ultimate particles, said fiocs under high magnification visually resembling grape clusters, the individual grapes of which are representative of ultimate particles of siliceous material, the average size of the ultimate particles being between 0.025 and 0.25 micron, at least 75 percent by weight of the pigment flocs being below 7 microns and the preponderant weight of these fiocs being in the range of 0.5 to 7 microns in size.

14. Finely divided precipitated fiocculated siliceous pigment containing metal oxide other than Si and having 1 to 150 moles of SiO per mole of its other metal oxide content, the pigment being in the form of fiocs of ultimate particles, said flocs under high magnification visually resembling grape clusters, the individual grapes of which are representative of ultimate particles of siliceous material, the average size of the ultimate particles being less than 0.25 micron, at least 55 percent by weight of the pigment fiocs being below 7 microns.

l5. Finely divided precipitated fiocculated siliceous pigment containing metal oxide other than SiO and having 1 to 150 moles of SiO per mole of its other metal oxide content, the pigment being in the form of fiocs of ultimate particles, said fiocs under high magnification visually resembling grape clusters, the individual grapes of which are representative of ultimate particles of siliceous material, the average size of the ultimate particles being less than 0.25 micron, at least 70 percent by weight of the pigment fiocs being below 7 microns.

16. Paper having dispersed therein finely divided precipitated fiocculated siliceous pigment containing metal oxide other than SiO having 1 to 150 moles of SiO per mole of its other metal oxide content, the pigment being in the form of fiocs of ultimate particles, said fiocs under high magnification visually resembling grape I at least 55 percent by weight of the pigment fiocs being 18 clusters, the individual grapes of which are representative of ultimate particles of siliceous material, the average size of the ultimate particles being less than 0.25 micron,

below 7 microns.

17. Paper having dispersed therein finely divided precipitated fiocculated siliceous pigment containing metal oxide other than Si0 and having 1 to 150 moles of SiO per mole of its other metal oxide content, the pigment age size of the ultimate particles being less than 0.25

micron, at least 70 percent by weight of the pigment fiocs being below 7 microns.

18. A method of preparing paper which comprises forming paper from a pulp slurry containing finely divided, precipitated, fiocculated siliceous pigment, the pig ment being in the form of fiocs of ultimate particles, said fiocs under high magnification visually resembling grape clusters, the individual grapes of which are representative" of ultimate particles of siliceous material, the average size of the ultimate particles being less than 0.25 micron,

I cipitated, fiocculated siliceous pigment, the pigmentbeat least 55 percent by weight of the pigment fiocs being below 7 microns.

19. A method of preparing paper which comprises forming paper from a pulp slurry of finely divided preing in the form of fiocs of ultimate particles, said fiocs under high magnification 'visually resembling grape clusters, the individual grapes of which are representative of ultimate particles of siliceous material, the average size of the ultimate particles being less than 0.25 micron, at least 75 percent by weight of the pigment flocs being below 7 microns.

20. A method of preparing paper which comprises forming paper from a pulp slurry containing finely di vided, precipitated, fiocculated siliceous pigment, the pigment being in the form of fiocs of ultimate particles, said fiocs under high magnification visually resembling grape clusters, the individual grapes of which are representa tive of ultimate particles of siliceous material, the average size of the ultimate particles being between 0.025 and 0.25 micron, at least 75 percent by weight of the pigment fiocs being below 7 microns.

21. A method of preparing paper which comprises forming paper from a pulp slurry containing finely divided, precipitated, fiocculated siliceous pigment, the pigment being in the form of fiocs of ultimate particles, said fiocs under high magnification visually resembling grape clusters, the individual grapes of which are representative of ultimate particles of siliceous material, the average size of the ultimate particles being less than 0.25 micron, at least 55 percent by weight of the pigment fiocs being below 7 microns and the preponderant weight of these fiocs being in the range of 0.5 to 7 microns in size.

22. A method of preparing paper which comprises forming paper from a pulp slurry containing finely divided, precipitated fiocculated siliceous pigment, the pigment being in the form of fiocs of ultimate particles, said fiocs under high magnification visually resembling grape clusters, the individual grapes of which are representative of ultimate particles of siliceous material, the average size of the ultimate particles being less than 0.25 micron, at least 75 percent by weight of the pigment fiocs being below 7 microns and the preponderant weight of these fiocs being in the range of 0.5 to 7 microns in size.

23. A method of preparing paper which comprises forming paper from a pulp slurry containing finely divided, precipitated fiocculated siliceous pigment containing metal oxide other than SiO and having 1 to moles of SiO per mole of its other metal oxide content, the pigment being in the form of flocs of ultimate particles, said fiocs under high magnification visually resembling grape '19 clusters, the individual grapes of which are representative of. ultimate particles of siliceous material, the average size of-the ultimate particles being less than 0.25 micron;

at least.- 55 percent by weight of the pigmentflocs being below 7 microns.

.24. A method of preparing paper which comprises forming paper from a pulp slurry containing finely divided, precipitated flocculated siliceous pigment containing metal oxide other than SiQg and having 1 to 150 moles of SiO per mole of its other metal oxide content,

the pigment being'in the form of flocs of ultimate particles, said fiocs under high magnification visually resembling grape clusters, the individual grapes of which are representative of ultimate particles of siliceous material, the average size of the ultimate particles being less than 0.25 micron, at least 70 percent by weight of the pigment flocs being below 7 microns.

25. Finely divided, white, precipitated alumina-silica pigment having at least 6 moles of SiO per mole of A1 the pigment being in the form of flocs of ultimate particles, said flocs under high magnification visually resembling grape clusters, the individual grapes of which are representative of ultimate particles of siliceous mate-- rial, the size of the ultimate particles being below 0.1

micron, at least 55- percent by weight of the pigment flocs being below 7 microns.

;26. Finely divided;-white, precipitated alumina-silica pigment having at least 6 moles of SiO per mole of A1 0 the pigment being in the fornr of flocs of ultimate particles, said flocs under high magnification visually' resembling grape clusters, the individual grapes of which are representative of ultimate particles of siliceous material, the size of the ultimate particles being 0.05 to 0.1 micron, at least 70 to 95 percent by weight of the pigment flocs being below 7' microns.

27. Paper having dispersed therein finely divided,-

20 percent by weight: of the pigmentflocsbeing below 7 microns.

c 28. Paper having dispersed therein finely divided, 1 white, precipitated alumina-silica pigment "having at least 6 moles of SiO;; per mole of A1 0 the pigment being in" the form of flocs of ultimate particles, said flocs under high magnification visually resembling grape clusters,- theindividual grapes of which are representative of ultimate particles of siliceous material, the size of the ultimate particles being 0.05 to 0.1 micron, at least 70 to 95 percent by weight of the pigment fiocs being below 7 microns.

29; The pigment of "claim 25 wherein the pigment contains 6 to 100 moles of SiO per mole of A1 0 30. 'The pigment of claim 26 wherein the preponderantweight of the pigment flocs is microns in Y Y a 31. The paper of claim 27 wherein the silica pigment contains 6 to 100 moles of-Si0 per-mole of A1 0 32. The paper of claim 28 wherein the silica pigment contains 6 to 1-00 moles of SiO per mole of Al O 33. Finely divided, white, precipitated alumina-silica pigment having at least 6 moles of 'SiO per mole of A1 0 the. pigment being in the form of flocs of ultimate particles, saidflocs under'high magnification visually re,

sembling grape clusters, the individual grapes of which are representative of ultimate particles of siliceous mate rial, the size of the ultimate particles being below 0.25

micron, at least percent by weight of the pigment flocs being below 7 microns. I

-34. Finely divided, white, precipitated alumina-silica pigment having at least 6 moles of SiO per mole of A1 0 the pigment being in the form of fiocs of ultimate particles, said flocs under high magnification visually resembling grape clusters, the individual grapes of which are representative of ultimate particles of siliceous material, the size of the ultimate particles being 0.05 to 0.25

micron, at least to percent by Weight of thepigment fiocs being below 7 microns.

References Cited in the fileof this patent UNITED STATES PATENTS 2,805,955 Allen Sept. 10, 1957 in the range of 0.5 to 7 

6. PAPER HAVING DISPERSED THEREIN FINELY DIVIDED, PRECIPITATED, FLOCCULATED SILICEOUS PIGMENT, THE PIGMENT BEING IN THE FORM OF FLOCS OF ULTIMATE PARTICLES, SAID FLOCS UNDER HIGH MAGNIFICATION VISUALLY RESEMBLING GRAPE CLUSTERS, THE INDIVIDUAL GRAPES OF WHICH ARE REPRESENTATIVES OF ULTIMATE PARTICLES OF SILICEOUS MATERIAL, THE AVERAGE SIZE OF THE ULTIMATE PARTICLES BEING LESS THAN 0.25 MICRON, AT LEAST 55 PERCENT BY WEIGHT OF THE PIGMENT FLOCS BEING BELOW 7 MICRONS. 